K+ channels are key regulators of cell excitability in the nervous system, skeletal, smooth and cardiac muscle and secretory glands. Therefore, it is not surprising that dysfunction of K+ channels are the underlie cause of uncountable human pathologies, such as: neurological disorders, cardiac diseases and diabetes. For this reason, it is extremely important to understand at the atomic level the properties of K+ channels that determine cell excitability. Understanding ion selectivity, permeation and gating at atomic detail will allow us to identify highly-specific therapeutic agents that can recognize with precision a specific channel's kinetic state that need to be regulated to correct a given channelopathy. It follows that for two decades, functional, structural and computational studies, performed on the KcsA-closed structure, have improved our understanding of how the structure defines the function of K+ channels. Recently, we have made two important scientific contributions: the first atomic-resolution description of KcsA's minimal kinetic cycle and the quantification of the energetics associated with each kinetic cycle reaction. However, important unanswered questions remain, mostly due to our inability to conduct simultaneous structural and functional studies in: 1 ) the open-state of the channel 2) mutants of the highly conserved glycine residues in the selectivity filter, which are known to affect inactivation gating, ion selectivity and/or ion binding in the closed and open states of the channel, 3) tandem-tetramers to dissect cooperativity of ion channel function, and 4) mutants that dissect the non-conductive open states of KcsA by precisely uncoupling activation-gate opening from the onset of ion permeation/inactivation at the selectivity filter. Consequently, we propose the following Specific Aims: 1) To characterize the structure-function correlations between the selectivity filter, ion occupancy and conduction properties of KcsA ?trapped? with its activation gate open 2) To determine the structure-function correlations of KcsA subunit cooperativity using tandem hetero-tetramers 3) To understand the role of KcsA's allosteric coupling on the onset of ion permeation, C-type inactivation and ion selectivity and 4) To understand the structural and functional roles of the glycine residues within the K+ channel selectivity filter. The novelty of our experimental approaches, together with our vast experience working with ion channels, fully qualifies us to perform the proposed project. Finally, the completion of this project will bring us closer to a complete atomistic understanding of ion-channel function, allowing us to identify ion-channels kinetic intermediates more suitable as pharmaceutical targets for the next generation of more specific and safer therapeutic drugs.